What is the “Memory Effect”?

The “memory effect” (also known as battery effect, lazy battery effect or battery memory) can be seen in Ni-Cad and NiMH batteries. It is caused by the user continually re-charging the battery before being it has fully discharged. It describes one very specific situation in which certain NiCd batteries gradually lose their maximum energy capacity if they are repeatedly recharged after being only partially discharged. The battery appears to “remember” the smaller capacity.
The source of the effect are changes of the characteristics of the underused active materials of the cell. is an alleged effect observed in nickel cadmium rechargeable batteries that causes them to hold less charge. The term is commonly misapplied to almost any case in which a battery appears to hold less charge than was expected. These cases are more likely due to battery age and use, leading to irreversible changes in the cells due to internal short-circuits, loss of electrolyte, or reversal of cells. The battery will eventually ‘forget’ the unused capacity.
The way to avoid the “memory effect” is to fully charge and discharge the battery at least once every month. This will ensure the battery remains healthy. Other problems perceived as memory effect

Phenomena which are not true memory effect may also occur in other battery types than sintered-plate nickel-cadmium cells.

Temporary effects
- Voltage depression due to long-term over-charging
  A common process often ascribed to memory effect is voltage depression. In this case the peak voltage of the battery drops more quickly than normal as it is used, even though the total energy remains almost the same. In modern electronic equipment that monitors the voltage to indicate battery charge, the battery appears to be draining very quickly. To the user it appears the battery is not holding its full charge, which seems similar to memory effect. This is a common problem with high-load devices such as digital cameras. Voltage depression is caused by repeated over-charging of a battery, which causes the formation of small crystals of electrolyte on the plates. These can clog the plates, increasing resistance and lowering the voltage of some individual cells in the battery. This causes the battery as a whole to seem to discharge rapidly as those individual cells discharge quickly and the voltage of the battery as a whole suddenly falls. This effect is very common, as consumer trickle chargers typically overcharge.
- Repair
  The effect can be overcome by subjecting each cell of the battery to one or more deep charge/discharge cycles. This must be done to the individual cells, not a multi-cell battery; in a battery some cells may discharge before others, they are then subjected to a reverse charging current by the remaining cells, causing irreversible damage.
  One way to avoid the dreaded “memory effect” is to fully cycle (fully charge and then fully discharge) your battery at least once every two to three weeks. Batteries can be discharged by unplugging the device’s AC adapter and letting the device run on the battery until it ceases to function. This will ensure the battery remains healthy and maintains its full capacity during its lifespan.
- Charger cutoff voltage too high
  An incorrect charger setting coupled with a slight voltage depression can cause a battery to be identified as “dead” even when nearly the full capacity remains usable (albeit at a slightly reduced voltage).
- High temperatures
  High temperatures reduce the charge accepted by the cells and the voltage charged to.
- Other causes
  - Operation below 0 °C
  - High discharge rates (above 5C) in a battery not specifically designed for such use
  - Inadequate charging time
  - Defective charger
Permanent loss of capacity
- Deep discharge
  Some rechargeable batteries can be damaged by repeated deep discharge. Batteries are composed of multiple similar, but not identical, cells. Each cell has its own charge capacity. As the battery as a whole is being deeply discharged, the cell with the smallest capacity may reach zero charge and will “reverse charge” as the other cells continue to force current through it. The resulting loss of capacity is often ascribed to the memory effect.
- Age and use—normal end-of-life
  All rechargeable batteries have a finite lifespan and will slowly lose storage capacity as they age due to secondary chemical reactions within the battery whether it is used or not. Some cells may fail sooner than others, but the effect is to reduce the voltage of the battery. Lithium-based batteries have one of the longest idle lives of any construction, and examples abound that are nearly 20 years old which exhibit almost their as-new capacity[citation needed]. Unfortunately the number of operational cycles is still quite short and the best examples rarely last more than 500 or so complete charge/discharge cycles. The lifetime of lithium batteries decreases with temperature and state of charge, whether used or not; maximum life of lithium cells not in use is achieved by refrigerating (without freezing) charged to 40%.